Conventionally, LiCoO2 is mainly used as a positive active material for a nonaqueous electrolyte secondary battery. However, the discharge capacity of LiCoO2 is about 120 to 130 mAh/g.
A solid solution of LiCoO2 and another compound is known as a material of a positive active material for a nonaqueous electrolyte secondary battery. Li[Co1−2xNixMnx]O2 (0<x≦½), which has a crystal structure of an α-NaFeO2 type and is a solid solution of three components, LiCoO2, LiNiO2 and LiMnO2, has been presented in 2001. LiNi1/2Mn1/2O2 or LiCo1/3Ni1/3Mn1/3O2 as an example of the solid solution has a discharge capacity of 150 to 180 mAh/g and is also superior in charge-discharge cycle performance.
For the above-mentioned so-called “LiMeO2 type” active material, the so-called “lithium excess type” active material, in which a compositional ratio Li/Me of lithium (Li) to a transition metal (Me) is more than 1 and for example Li/Me is 1.25 to 1.6, is known. A compositional formula of such a material can be denoted by Li1+αMe1−αO2 (α>0). Here, when the compositional ratio Li/Me of lithium (Li) to a transition metal (Me) is denoted by β, β=(1+α)/(1−α), and thus α=0.2 if Li/Me is 1.5, for example.
In Patent Document 1, an active material, which is a kind of such an active material and can be represented as a solid solution of three components of Li[Li1/3Mn2/3]O2, LiNi1/2Mn1/2O2 and LiCoO2, is described. Further, as a method of manufacturing a battery using the above-mentioned active material, it is described that by providing a production step in which charge at least reaching a region where a potential change is relatively flat, occurring within a positive electrode potential range of more than 4.3 V (vs. Li/Li+) and 4.8 V (vs. Li/Li+) or less, is performed, it is possible to manufacture a battery which can achieve a discharge capacity of 177 mAh/g or more even when employing a charge method in which a maximum achieved potential of a positive electrode at the time of charge is 4.3 V (vs. Li/Li+) or less.
Such a so-called “lithium-excess type” positive active material has a problem that charge-discharge cycle performance and high rate discharge performance are not sufficient. Further, as described above, when in at least the first charge, charge is performed up to a relatively high potential more than 4.3 V, particularly up to a potential of 4.4 V or more, there is a feature of achieving a high discharge capacity, but initial charge-discharge efficiency (hereinafter, referred to as initial efficiency) in this case is not adequately high. Moreover, in Patent Document 1, the stability of the crystal structure, the oxygen position parameter, the specific surface area, and the tapped density are not described.
In Patent Document 2, it is described that a lithium-containing metal composite oxide of a layered rock salt type, an oxygen position parameter and a distance between lithium and oxygen relate to an initial discharge capacity and charge-discharge cycle performance. However, it is not described how the oxygen position parameter affects the high rate discharge performance.
In Patent Documents 3 and 4, an active material for a lithium secondary battery of the general formula xLiMO2.(1−x)Li2M′O3 (0<x<1) is described, and it is also described that M is at least one selected from Mn, Co and Ni, and Mn is selected for M′, and it is shown that the active material containing enriched Li stabilizes a crystal structure, and by using the active material, a lithium secondary battery having a large discharge capacity is attained, but the stability of the crystal structure in being electrochemically oxidized to a high potential is not clear, and improvements in charge-discharge cycle performance, initial efficiency and high rate discharge performance are not described. Also, in these Patent Documents, an active material in which the content of Mn is large and the content of Co is small is not specifically described, and the oxygen position parameter, the specific surface area, and the tapped density are not also described.
In Patent Document 5, an active material for a lithium secondary battery of the general formula Li1+xNiαMnβAγO2 (x is 0 to 0.2, α is 0.1 to 0.5, β is 0.4 to 0.6, and γ is 0 to 0.1) is described, and it is also described that Co is selected for A, and it is shown that by using the active material having the above composition and containing enriched Li, which is produced by a specific method, a lithium secondary battery having a large discharge capacity is attained, but improvements in charge-discharge cycle performance, initial efficiency and high rate discharge performance are not described. Further, in Patent Document 5, the invention of an active material in which a molar ratio of Li to a transition metal element is 0.2 or more and the content of Mn in the transition metal element is more than 0.6 is not described, and the stability of the crystal structure, the oxygen position parameter, the specific surface area, and the tapped density are not also described.
In Patent Document 6, described is “A lithium battery, wherein when at least one transition metal selected from Groups 7A and 8A of the periodic table is denoted by Me, a transition metal different from the Me is denoted by Mt, and at least one element selected from the group consisting of Mt, Na, K, Rb, Cs, Al, Ga, In, Tl, B, Mg, Ca, Sr, Ba and Pb is denoted by A, the battery includes a positive active material comprising a composite oxide having the composition represented by LiXMeYA(1−Y)O(1+X) (1.3≦X≦2.5, 0.5≦Y≦0.999), and a hexagonal crystal structure” (claim 1), and it is shown that the positive active material containing enriched Li stabilizes a crystal structure, and by using the positive active material, a lithium secondary battery having a high energy density is attained, but the stability of the crystal structure in being electrochemically oxidized to a high potential is not clear, and improvements in charge-discharge cycle performance, initial efficiency and high rate discharge performance are not described. Further, in Patent Document 6, an active material in which x is 1.3, Me is Mn, A is Co, the content of Mn is large and the content of Co is small is described, but it is not specifically described that Co and Ni are selected as A, and the oxygen position parameter, the specific surface area, and the tapped density are not also described.
In Patent Document 7, described is the invention of “A positive electrode material for a nonaqueous electrolyte secondary battery using a lithium manganese nickel cobalt oxide comprising lithium, manganese, nickel, cobalt and oxygen, wherein the lithium manganese nickel cobalt oxide has a layered structure and is represented by Li[Li[(1−2x−y)/3]NixCoyMn[(2−x−2y)/3]]O2, and x and y satisfy 0.2<x<0.5, 0<y<0.2, and 1<2x+y” (claim 1), and it is shown that by using the positive electrode material, cycle characteristics are improved, but improvements in initial efficiency and high rate discharge performance are not described. Further, as an example, Li1.15Ni0.25Co0.05Mn0.55O2, a lithium manganese nickel cobalt oxide in which a molar ratio Li/Me of Li to all transition metal elements Me is 1.353, a molar ratio Co/Me is 0.059, and a molar ratio Mn/Me is 0.647, is described (Example 4), but the stability of the crystal structure of the positive electrode material, the oxygen position parameter, the specific surface area, and the tapped density are not described.
In Patent Document 8, described is “A positive active material containing a lithium composite oxide represented by the following chemical formula: [Chem. 1] Li1+a[MnbCocNi(1−b−c)](1−a)O(2-d), wherein a, b, c and d satisfy 0<a<0.25, 0.5≦b<0.7, 0≦c<(1−b), and −0.1≦d≦0.2” (claim 1), and it is shown that by using the positive active material, a large discharge capacity and good cycle characteristics can be realized, and charge-discharge efficiency is also shown, but the invention is not intended to improve the charge-discharge efficiency, and an improvement in high rate discharge performance is not described. Further, as an example, lithium composite oxides in which a molar ratio Li/Me of Li to all transition metal elements Me is 1.30 are described (Example 1-3, Examples 2-2 to 2-8, Examples 3-1 and 3-2), but these composite oxides are synthesized by using “a solid state method”, and most of the composite oxides contain less Mn. As only one active material containing Mn in a large amount, Li1.13[Mn0.65Co0.20Ni0.15]0.87O2 is shown, but the stability of the crystal structure of the positive active material, the oxygen position parameter, the specific surface area, and the tapped density are not described.
On the other hand, with respect to lithium transition metal composite oxides composed of Li and transition metal elements (Co, Ni, Mn, etc.), active materials in which the specific surface area and the tapped density are increased are known (refer to e.g., Patent Documents 9 and 10).
In Patent Document 9, described is the invention of “A positive active material having a laminar crystal structure, wherein a sequence of a lithium element and an oxygen element composing an oxide composed of crystal particles of the oxide containing at least three transition metal elements is a cubic structure, and a specific surface area is 0.9 to 2.5 m2/g, and a tapped density is 1.8 to 2.5 g/cm3” (claim 1), and it is shown that in accordance with this invention, a lithium secondary battery having high initial charge-discharge efficiency (initial efficiency) and excellent durability of a charge-discharge cycle can be obtained. Further, in Patent Document 9, described is “A positive active material for a lithium secondary battery represented by the general formula Li[LiqCoxNiyMnz]O2, in which q satisfies −0.2≦q≦0.2, 0.8≦1+q≦1.2, X satisfies 0.1<X≦0.6, Y satisfies 0.1<Y≦0.6, Z satisfies 0.2<Z≦0.6, and X, Y and Z satisfy 0.7≦X+Y+Z≦1.2” (claim 2), but since a positive active material, in which a molar ratio of Li to the transition metal element is 1.2 or more and a molar ratio of Mn in the transition metal element is 0.625 or more, is not specifically described, the initial efficiency and the charge-discharge cycle performance of the positive active material having such composition cannot be predicted. Further, the stability of the crystal structure composing the positive active material and the oxygen position parameter are not described.
In Patent Document 10, described is the invention of “A lithium nickel manganese cobalt composite oxide for a lithium secondary battery positive active material represented by the following general formula (1):LixNi1−y−zMnyCozO2  (1)
in which x satisfies 0.9≦x≦1.3, y satisfies 0<y<1.0, z satisfies 0<z<1.0, and y and z satisfy y+z<1, wherein an average particle size is 5 to 40 μm, a BET specific surface area is 5 to 25 m2/g, and a tapped density is 1.70 g/ml or more” (claim 1), and it is shown that in accordance with this invention, a lithium secondary battery having high initial efficiency and excellent loading characteristics (high rate discharge performance) can be obtained. However, since a positive active material in which x is 1.2 or more and y is 0.625 or more is not specifically described in Patent Document 10, the initial efficiency and the high rate discharge performance of the positive active material having such composition cannot be predicted. Further, the stability of the crystal structure composing the positive active material and the oxygen position parameter are not described.
In improvement in high rate discharge performance of the lithium transition metal composite oxide in which lithium is enriched, partial fluorination of a part of oxygen (Non-patent Document 1) and a surface coating technology (Non-patent Document 2) are proposed. However, all of these are technologies expecting use at 4.5 V or more as a positive electrode charge potential corresponding to a potential region of decomposition of an electrolyte solution, and are not technologies intended to improve high rate discharge performance at the time when the positive electrode charge potential is changed to a potential lower than 4.5 V, for example, 4.3 V, after initial formation to use a battery.
Moreover, the so-called “lithium-excess type” positive active material has a problem that oxygen gas is generated during charge (refer to e.g., Patent Documents 11, 12, and Non-patent Documents 3, 4).
In Patent Document 11, described is the invention of “A method of manufacturing an electrochemical element comprising the step of charging an electrode active material having a plateau potential, at which gas is generated in a charge range, to the plateau potential or more; and the step of removing the gas” (claim 1), “The manufacturing method according to any one of claims 1 to 3, wherein the positive active material has a plateau potential of 4.4 to 4.8 V” (claim 4), “The manufacturing method according to claim 1, wherein the gas is oxygen (O2) gas” (claim 5), “An electrochemical element, wherein an electrode active material having a plateau potential at which gas is generated in a charge range is charged to the plateau potential or more, and then the gas is removed” (claim 6), “The electrochemical element according to claim 6, wherein the electrode active material has a plateau potential of 4.4 to 4.8 V” (claim 7), and “The electrochemical element according to claim 8, wherein after charging to the plateau potential or more and removing gas, a discharge capacity of the electrode active material ranges from 100 mAh/g to 280 mAh/g in a voltage range of 3.0 to 4.4 V” (claim 10).
Further, when as the above electrode active material, a chemical formula 1 “a solid solution of XLi(Li1/3M2/3)O2+YLiM′O2 in which M is one or more elements selected from metals having an oxidation number of 4+, M′ is one or more elements selected from transition metals, and X and Y satisfy 0<X<1, 0<Y<1, and X+Y=1” (claim 2, claim 8, and paragraph [0024]) is used, “When being charged to an oxidation-reduction potential of M′ or more, Li is extracted and simultaneously oxygen is also detached in order to have a balance between oxidation and reduction. Accordingly, the electrode active material has a plateau potential” (paragraph [0025]), “The compound of the chemical formula 1 is preferred since the electrode active material functions stably as an electrode active material in a charge-discharge cycle after the electrode active material is charged to a charge voltage (4.4 to 4.8 V) of a plateau potential or more and the gas removal step is performed” (paragraph [0026]), and “Preferably, M is one or more elements selected from Mn, Sn and Ti metals, and M′ is one or more elements selected from Ni, Mn, Co and Cr metals” (paragraph [0027]) are described.
Moreover, in Patent Document 11, “When a battery is configured by a method in which the active material is charged to a plateau potential or more once or more and then the gas removal step is performed according to the present invention, even though the active material is charged to a plateau potential or more continuously, a battery having a high capacity is configured, and a problem of a battery due to gas generation can also be solved. That is, After charging to a plateau potential or more, gas is not generated in charge in the subsequent cycles, and a plateau range disappears (refer to FIG. 4) (paragraph [0022]) is described, and as Example 4, it is shown that in the case where Li(Li0.2Ni0.2Mn0.6)O2(⅗[Li(Li1/3Mn2/3)O2]+⅖[LiNi1/2Mn1/2]O2) is used as a positive active material (paragraph [0048]), and “charged to 4.8 V in a first cycle, and charged to 4.4 V in a second cycle” (paragraph [0060]), a battery having a high capacity can be obtained (refer to FIG. 5). However, it is suggested that as described in Comparative Examples 5 and 6, oxygen gas is not generated when charging up to 4.25 V or 4.4 V that is a plateau potential or less, but only a battery having a low discharge capacity can be obtained (paragraph [0056], FIG. 1, paragraph [0057], and FIG. 2), and therefore it cannot be said that a positive active material, which does not generate oxygen gas even when charging up to a voltage higher than a plateau potential, is shown.
In Non-patent Documents 3 and 4, it is shown that when Li[NixLi(1/3−2x/3)Mn(2/3−x/3)]O2 is used as a positive active material, oxygen gas is generated at a charge voltage (4.5 V to 4.7 V) that is a plateau potential or more (left column line 4 to right column line 2 in page A818 in Non-patent Document 3, left column line 9 in page A785 to right column line 4 in page A788 in Non-patent Document 4), but it is not shown that oxygen gas is not generated when charging up to a high voltage that is a plateau potential or more.
In Patent Document 12, described is the invention of “A nonaqueous-type (lithium ion) secondary battery formed by winding or layer stacking a positive electrode plate, in which a current collector is provided with an active material layer capable of intercalation/deintercalation lithium ions thereon, and a negative electrode plate with a separator sandwiched between the electrode plates to form an electrode group, and housing the electrode group in a case hermetically together with a nonaqueous electrolyte, wherein an active material to be charged at 4.3 V or less on the Li/Li+ basis and a substance to generate oxygen gas at the time of overcharge exist on the positive electrode plate” (claim 1), and “The lithium ion secondary battery according to claim 1, wherein an active material represented by Li[(Ni0.5Mn0.5)xCoy(Li1/3Mn1/3)z]O2 (x+y+z=1, z>0) or LiαNiβMnγO2 (α is 1.1 or more, β:γ=1:1) is used as the substance to generate oxygen gas at the time of overcharge” (claim 2), and it is described that the lithium-excess transition metal composite oxides (Examples 1 to 3 and 7: Li1.2Ni0.4Mn0.4O2, Examples 4 to 6: Li[(Ni0.5Mn0.5)1/12Co1/4(Li1/3Mn2/3)1/3]O2 (x= 5/12, y=¼, z=⅓)) easily generate oxygen gas at the time of overcharge in comparison with the lithium transition metal composite oxide (not lithium-excess) (Comparative Examples 1: LiCoO2, Comparative Example 2: LiNi0.5Mn0.5O2, Comparative Example 3: Li(N1/3Mn1/3Co1/3)O2) (paragraph [0064]), and it is described that in the invention described in Patent Document 12, on the contrary, utilizing the above-mentioned property of the lithium-excess transition metal composite oxide, “By gas generation at the time of overcharge, since the positive active material layer is detached from the current collector, the positive electrode plate is detached from the separator, or positive electrode layer inside is split, it is possible to cut out charge and prevent decomposition of the electrolyte solution, decomposition of the positive active material and short-circuit due to deposit of Li to the negative electrode side” (paragraph [0010]).
Moreover, since it is described in Patent Document 12 that “In the lithium ion secondary battery of the present invention, as the positive active material generating oxygen gas at the time of overcharge, it is preferred to use a lithium-excess positive active material Li[(Ni0.5Mn0.5)xCoy(Li1/3Mn1/3)z]O2 (wherein x+y+z=1, z>0) or an active material represented by LiαNiβMnγO2 (α is 1.1 or more, β:γ=1:1). When the above active material is used, oxygen gas is generated at about 4.5 V on the Li/Li+ basis, and thereby, a distance between the positive electrode and the negative electrode can be increased”, the positive active material not generating oxygen gas when charging up to 4.5 V or more is not shown.
On the other hand, since oxygen gas generated from the positive active material causes failures such as oxidation of a solvent constituting an electrolyte of a nonaqueous electrolyte secondary battery (lithium secondary battery) and heating of the battery, a positive electrode (positive active material) for a nonaqueous electrolyte secondary battery in which oxygen gas generation at the time of overcharge or high-temperature is suppressed is also developed (refer to e.g., Patent Documents 13 and 14).
With respect to the invention described in Patent Document 13, in claim 1, difficulty of generation of oxygen gas of a lithium-containing composite oxide (lithium transition metal composite oxide) is specified as “a local maximum value of oxygen generation peak in gas chromatography-mass spectrometry measurement of the composite oxide” and “a range of 330 to 370° C.”, and it is described that “In GC/MS measurement, a temperature of a positive composite is raised at a rate of 10° C./min from room temperature to 500° C., and behavior of oxygen generation was observed. Here, the obtained oxygen generation spectrum (A) is shown in FIG. 3. As is apparent from FIG. 3, in the spectrum (A), a local maximal value of an oxygen generation peak is positioned at a side of temperature higher than 350° C. From this, it is evident that the positive active material of the present invention is hardly decomposed while generating oxygen, and is extremely superior in stability even when being exposed to high-temperature in overcharge region of a battery voltage of 4.7 V” (paragraph [0041]).
However, in Patent Document 13, only the positive active material “represented by the general formula: LizCo1−x−yMgxMyO2, wherein an element M in the general formula is at least one selected from the group consisting of Al, Ti, Sr, Mn, Ni and Ca, and x, y and z in the general formula satisfy 0≦z≦1.03, 0.005≦x≦0.1, and 0.001≦y≦0.03” is specifically described (claims 2 and 3), and the “lithium-excess type” positive active material not generating oxygen gas in an overcharge region is not shown.
In Patent Document 14, it is shown that release of oxygen from a positive electrode at the time of high-temperature is suppressed by mixing an oxygen-storing material with a positive active material or attaching the oxygen-storing material to the positive active material (claim 1, paragraphs [0005], [0056]), and it is described that “Peak temperatures of oxygen detachment of the positive electrodes (samples 1 to 4) having a Ce oxide or a Ce—Zr oxide as an oxygen-storing material are all 300° C. or higher, and the peak temperatures were significantly increased relative to that in the sample 5 not having an oxygen-storing material. This shows that in the samples 1 to 4, a phenomenon of releasing oxygen from the positive electrode is suppressed better (to higher temperature) than the phenomenon in the sample 5” (paragraph [0057]), but the positive active material (lithium transition metal composite oxide) not generating oxygen gas at a high-temperature is not shown.